For creating custom ground markings, I use Gimp. I basically create what I want, like "RWY AHEAD" for example. I make sure it follows the guidline of the SDK Documentation, and import it into my own MaterialLib asset in the project.
If you are want to create a custom center line however, my tip would be to:
Turn on center line marking in your Runway object (To use it as a guide)
Add a PaintedLine object, with "EDGE_SERVICE_SOLID" as type
Place the PaintedLine with desired length using the Runway default center line as a guide to make it straight and follow the Runway,
When you are done, you can disable the center line marking on you Runway object
Hope this helps!